Much effort has been expended in studies and experimentation for stabilizing medium and heavy lift helicopters and the suspended containerized load for off-shore unloading of a slung load. One objective is to accelerate the unloading operation, particularly when harbor facilities are inadequate or nonexistent. Also guidelines have been suggested for carrying a suspended load faster, easier and more safely. In addition, display system for monitoring the load's position as it is being lowered are being studied. It is rightfully assumed that no amount of electronic stabilization equipment and of pilot effort is capable of providing safety and precision to the unloading operation when the ship is tossing about in rough sea. Reference is made to Army Electronics Command Report of Jan-1973, number AD-756865 "Simulation of Helicopter Containership Loading" which identified the problems. A loading platform kept level on the ship's deck is suggested as a simple solution to a complex problem.
The best accuracy that can be accomplished in precisely positioning a load using ground referenced position, velocity, and acceleration displays and aircraft feedback controls is 1.5 feet in radial ground position, using random wind disturbances of 6ft/sec rms. (US ARMY ECOM-4334 Technical Report AD-A012813 by CM. Tsoubanos dated July 1975). However, placement of a container on a containerized ship or truck bed requires .+-.4 in. accuracy. It is obvious existing techniques for unloading cargo onto a platform are unsatisfactory even when the platform is stationary. If the loading platform is large enough to accommodate a 1.5 feet radial error and is essentially stationary, then the unloading task from a helicopter can be satisfactorily performed. The techniques described in this invention will enable a safe unloading of an aircraft suspended load onto the deck of a ship at sea.